1. Field of the Invention
This invention relates to ferroelectric and high dielectric transistors, and more particularly to such transistors utilizing layered superlattice materials and tantalum oxide materials.
2. Statement of the Problem
It has been postulated for at least 40 years that it may be possible to design a memory in which the memory element is a ferroelectric field effect transistor (FET). See Shu-Yau Wu, xe2x80x9cA New Ferroelectric Memory Device, Metal-Ferroelectric-Semiconductor Transistorxe2x80x9d, IEEE Transactions On Electron Devices, pp. 499-504, August 1974; and S.Y. Wu, xe2x80x9cMemory Retention and Switching Behavior Of Metal-Ferroelectric-Semiconductor Transistorsxe2x80x9d, Ferroelectrics, Vol. 11, pp. 379-383, 1976. Because the ferroelectric effect measured in the early devices of Wu was only a temporary, single state effect rather than a long lived two-state effect, it is now believed that this effect was charge injection effect rather than an effect due to ferroelectric switching. Moreover, the polarizability of the bismuth titanate thin films made by Wu had a polarizability of only 4 microcoulombs per centimeter squared (xcexcC/cm2). This is much too low to make an effective memory. See European Patent application 0489519 A2 published Jul. 6, 1992, page 2, lines, 4-6. The polarizability of bismuth titanate was known at that time to be far higher than any other layered superlattice material. See M. E. Lines and A. M. Glass, Principles and Applications of Ferroelectrics And Related Materials, Clarendon Press, Oxford, 1977, Appendix F, pp. 624 and 625. As a result, no one ever again attempted to use a layered superlattice material in an actual memory utilizing a ferroelectric FET as the memory element. In fact, at the present time, no commercial memory in which the memory element is a ferroelectric FET has yet been made. Because such a memory, if feasible, would be non-volatile, dense, and faster than any present non-volatile memory now available, it would be highly desirable if a ferroelectric memory utilizing a ferroelectric memory element were available.
If a commercially useful ferroelectric FET could be made, a similar FET could be useful as a switching element in other electronic devices also, such as a select transistor in a DRAM or FERAM.
The invention solves the above problem by providing a transistor including a layered superlattice material. Preferably, the charge storage element of the transistor comprises a layered superlattice material.
In one embodiment, the invention provides a ferroelectric FET in which the charge storage element is a ferroelectric layered superlattice material. The ferroelectric FET may be a MFMISFET (metal/ferroelectric/metal/insulator/semiconductor FET), a MFSFET (metal/ferroelectric/semiconductor FET), or any other form of ferroelectric FET. As known in the art, the term xe2x80x9cmetalxe2x80x9d in the names for the above FETs does not mean that a metal must be used for the layer indicated by xe2x80x9cmetalxe2x80x9d, but only that the layer must be conductive. For example, polysilicon is often used as the xe2x80x9cmetalxe2x80x9d in such FETs.
In another embodiment, the invention provides a field effect transistor in which the charge storage element is a non-ferroelectric high dielectric constant layered superlattice material. The field effect transistor is preferably the select transistor in a memory cell, which is preferably a DRAM or an FERAM.
The invention not only provides a ferroelectric memory in which the memory cell is a ferroelectric FET and which demonstratably works, but also provides one that is dense, and manufacturable. It also provides a select transistor for a DRAM or FERAM having a high dielectric constant storage insulator. A device in accordance with the invention preferably includes an interface insulator located between the channel and the layered superlattice material of a FET. Other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.